1. Field of the Invention
The present invention relates to an ultrasonic motor, and more particularly, to a method for controlling the rotation speed of an ultrasonic motor which is driven by elastic vibration caused by the piezoelectric effect of a piezoelectric member.
2. Description of the Related Art
In recent years, ultrasonic motors have been received much attention. An ultrasonic motor has a vibrator comprising a piezoelectric member formed of piezoelectric ceramic or the like for converting electric energy into mechanical energy, and an elastic substrate such as a metal substrate. The ultrasonic motor is driven by elastic vibration of the vibrator which is caused by applying an AC voltage to the vibrator.
Referring to the figures, a conventional disc-type ultrasonic motor using a traveling wave of the vibrator will be now described. FIG. 16 is a cross-sectional view of a conventional ultrasonic motor 700. The ultrasonic motor 700 includes a vibrator 3 and a rotor 6. The vibrator 3 includes a disc-shaped elastic substrate 1 formed of an elastic material such as metal or ceramic, and a disc-shaped piezoelectric body 2 formed of ceramic provided on one of two main surfaces of the elastic substrate 1. On the other main surface of the elastic substrate 1, projection 1A is provided for enlarging the displacement in a circumferential direction of the vibrator caused by the vibration. The rotor 6 includes an elastic body 4 formed, for example, of metal or plastic and a friction body 5 formed of an anti-abrasion material. The friction body 5 enhances the resistance against abrasion and ensures stable contact between the rotor 6 and the projection 1A.
The vibration of the vibrator 3 is converted into a rotation of the rotor 6 so as to be transmitted through a transmission axis 7. The transmission axis 7 includes a protruding porion 7A for efficiently and stably transmitting rotational force of the rotor 6 to the transmission axis 7, and an output transmission portion 7B for outputting the rotational force. An elastic member 8 having a large frictional constant is provided on the protruding portion 7A. The elastic member 8 absorbs and compensates abnormal vibrations of the transmission axis 7 so that the rotational force of the rotor 6 is efficiently and stably transmitted.
The vibrator 3 is supported by a supporting member 9 provided on a base 13. The supporting member 9 holds the vibrator 3 without obstructing the vibration thereof. A bearing 10 is provided at the base 10 for positioning the transmission axis 7. The rotor 6 is placed on the vibrator 3 on the side of the friction body 5 and is pressed against the vibrator 3 as a result of a pressure applied by a pressing member 11 such as a spring or the like. The pressure is controlled by a pressing adjuster 12.
FIG. 17 shows an exemplary structure of driving electrodes provided on the piezoelectric body 2. Four flexural vibration travelling waves are excited in the vibrator 3 by virtue of the driving electrodes. In FIG. 17, A.sub.0 and B.sub.0 denote two sets of driving electrodes each of which consists of divided portions each corresponding to 1/2 of a wavelength of a travelling wave exited on the vibrator 3. C.sub.0 denotes a region corresponding to 1/4 the wavelength of the travelling wave in the vibrator 3. D.sub.0 denotes an electrode corresponding to 3/4 wavelength of the travelling wave in the vibrator 3. Accordingly, the sets A.sub.0 and B.sub.0 of driving electrodes are positionally shifted from each other in a circumferential direction by 1/4 of a wavelength (i.e. 90.degree.) of the travelling wave excited in the vibrator 3.
In the electrode sets A.sub.0 and B.sub.0, two adjacent portions are alternately polarized in opposite directions along a thickness-direction of the electrodes. The piezoelectric body 2 comes into contact with the elastic substrate 1 by an opposite side of that shown in FIG. 17. The opposite side has driving electrodes which are not divided.
When the two sets of the electrodes A.sub.0 and B.sub.0 are used, each divided portions are short-circuited as indicated by a shade in FIG. 17. When the two sets of electrodes A.sub.0 and B.sub.0 are respectively supplied with AC voltages V.sub.1 and V.sub.2 having a 90.degree. phase difference from each other as expressed by Equations (1) and (2), a flexural vibration travelling wave as expressed by Equation (3) is excited in the vibrator 3. EQU V.sub.1 =V.sub.0 .times.sin (.omega.t) (1) EQU V.sub.2 =V.sub.0 .times.cos (.omega.t) (2) EQU .xi.-.xi..sub.0 .times.{cos (.omega.t).times.cos (kx)+sin ( t).times.sin (kt)}=.xi..sub.0 .times.cos (.omega.t-kx) (3)
where V.sub.0 : the maximum amplitude of applied voltage; .omega.: angular frequency; t: time; .xi.: amplitude of the flexural vibration; .xi..sub.0 : the maximum amplitude of the flexural vibration; k: the wave number (=2.pi./.lambda.); and x: the position of the coordinate in the travelling direction of the waves.
From Equation (3), the travelling direction of the wave can be switched simply by changing the phase difference between the voltages V.sub.1 and V.sub.2 to +90 or -90. Thus, the rotation direction of the rotor 6 can easily be changed.
FIG. 18 is a schematic view for describing the operation principles behind the ultrasonic motor 700. A driving force is transmitted from the vibrator 3 to the rotor by way of the friction body 5. The interface between the vibrator 3 and the rotor shown in FIG. 18 is described by using a simplified linear model, although in practice it has a more complicated shape as will be appreciated.
When the flexural vibration travelling wave is excited on the vibrator 3, the given points on the surface of the vibrator 3 move along an ellipse having the line of apsides (long axis) w with a minor axis (short axis) u as in shown in FIG. 18. The rotor 6 contacts the vibrator 3 at the apsis of the ellipse (for example, point A) via the friction body 5. The rotor 6 contacts the vibrator 3 only at the crests of the flexural vibration travelling wave, and receives a horizontal displacement component of the vibrator 3 by the resultant friction, thereby moving in an opposite direction relative to the travelling direction of the flexural traveling wave at a moving velocity v expressed by Equation (4). EQU v=.omega..times.w (6)
Since the crests of the wave are continuously moving, the contact points A of the vibrator 3 and the rotor 6 also move continuously with time. Thus, the rotor 6 is smoothly driven to rotate. The rotation direction of the rotor 6 can easily be changed by changing the travelling direction of the flexural traveling wave as mentioned above.
FIG. 19 shows an equivalent circuit 710 of the rotor 3 of the ultrasonic motor 700. The equivalent circuit 710 is expressed by a capacitance 14 (electric capacitance C.sub.e), a transformer 15 (transforming coefficient N) for transforming electric energy to mechanical energy, a capacitance 16 (mechanical elastic constant C.sub.m), a coil 17 (mass L.sub.m), and a resistance 18 (mechanical loss R.sub.m).
When a voltage V is applied to the piezoelectric member 2, a total current 19 (i) flows into the piezoelectric member 2 based on the frequency and magnitude of the amplitude of the voltage V. The total current 19 is divided into an electric branch current 20 (i.sub.e) flowing into the capacitance 14 and a mechanical branch current 21 (i.sub.m) flowing into the transformer 15. The mechanical branch current 21 is converted linearly by the transformer 15 into a displacement velocity v.sub.d of the vibrator 3 in a circumferential direction. The displacement velocity v.sub.d in the circumferential direction is expressed by Equation (5): EQU v.sub.d =d.xi..sup.' /dt (5)
where, .xi.' denotes a displacement of the vibrator 3 in the circumferential direction. Accordingly, by detecting the total current 19 or the mechanical branch current 21, the displacement .xi.' of the vibrator 3 in a circumferential direction is obtained by using an appropriate proportional constant.
As mentioned above, the rotational speed (moving velocity) v of the rotor 6 is proportional to an instant value of the amplitude of the flexural vibration of the vibrator 3, and the instant value of the amplitude of the flexural vibration (the displacement velocity) is proportional to the mechanical branch current 21 of the piezoelectric member 2 included in the vibrator 3. Accordingly, information of the rotational speed of the ultrasonic motor 700 can be obtained by detecting the mechanical branch current 21 flowing in the piezoelectric member 2.
FIG. 20 is a block diagram of a conventional driving circuit 720 for driving and controlling the speed of the ultrasonic motor 700.
A voltage-controlled oscillator 22 generates an AC signal 721 for driving the ultrasonic motor 700. The AC signal 721 from the voltage-controlled oscillator 22 is divided into two signals. One signal is shifted in phase by a predetermined amount (by +90.degree. or -90.degree.) by a phase shifter 23 and a resultant phase shifted signal 722 is input to a power amplifier 24. The power amplifier 24 amplifies the signal 722 to a level which is sufficiently high to drive the ultrasonic motor 700 based on supply power supplied from a direct current (DC) power supply 25. The other signal is directly input from the voltage-controlled oscillator 22 to another power amplifier 26. The power amplifier 26 amplifies the signal to a level which is sufficiently high to drive the ultrasonic motor 700 based on power supply supplied from the DC power supply 25. Then, the waveform of the amplified signal 723 from the power amplifier 24 is shaped by a coil 27, and then the shaped signal 724 is input to one driving electrode set 28 on the piezoelectric member 2 (not shown in FIG. 20). The signal 724 is also coupled to a capacitor 29 which has an equivalent capacitance of the piezoelectric member 2 under the electrode set 28. Similarly, the waveform of the amplified signal 725 from the power amplifier 26 is shaped by a coil 30, and then the shaped signal 726 is coupled to the other driving electrode set 31 on the piezoelectric member 2. The signal 726 is also coupled to a capacitor 32 which has an equivalent capacitance of the piezoelectric member 2 under the electrode set 31.
By applying the pair of the signals 724 and 726 to the electrode sets 28 and 31, respectively, a traveling wave of a flexural vibration is excited on the vibrator 3. The rotor 6 is rotated in the opposite direction of the flexural traveling wave, as mentioned above, so that the ultrasonic motor 700 is driven. In the piezoelectric member 2, a total current i flows, which depends on the frequency and the magnitude of a voltage signal applied to the piezoelectric member 2 according to the signals 724 and 726. The total current i is linearly converted into a voltage signal 727 by a resistance 33 coupled to ground. The voltage signal 727 is then applied to an input of a subtracter 34.
Electric currents i.sub.e ' equivalent to the electric branch current i.sub.e which flows in the electrode sets 28 and 31 depending on the frequency and the absolute value of the amplitude of the voltage signal applied to the piezoelectric member 2, are input to capacitors 29 and 32. These currents are applied together to a resistance 35 so as to be converted linearly into a voltage signal 728. The signal 728 is applied to another input of the subtracter 34. The output of the subtracter 34 is a voltage signal 729 which is proportional to the mechanical branch current e.sub.m and also proportional to the rotational speed of the rotor 6, as discussed above.
The output signal 729 of the subtracter 34 is converted into a DC voltage signal 730 by a rectifier 36, and is input to a microcomputer 37. The microcomputer 37 compares a value of the DC voltage signal 730 with data stored in the memory 38 so as to identify a current rotational speed of the ultrasonic motor 700. The data stored in the memory 38 is a predetermined (average) relationship between a rotational speed signal and a corresponding rotational speed. The microcomputer 37 then compares the current rotational speed with a target speed signal 732 given by a speed setting unit 39 so as to output a control signal 733 to the voltage-controlled oscillator 22. The frequency of the driving signal 721 is controlled so as to adjust the current rotational speed of the ultrasonic motor 700 to the target speed 732.
As a conventional ultrasonic motor, a disk-shape ultrasonic motor 700 using a traveling wave is described above. There is another type of ultrasonic motor having a ring-shape and which uses flexural vibration travelling waves having vibration modes of first or greater order in a radial direction and third or greater order in a circumferential direction. The ring-shaped ultrasonic motor operates by a similar principle and is driven by a similar driving circuit as mentioned above. A conventional standing wave ultrasonic motor in which the rotor is loaded by a vibration piece to rotate, also uses a similar driving system.
The conventional circuits for driving and controlling an ultrasonic motor such as the above-described driving circuit 720 have the following problems:
The relationship between a rotational speed signal for indicating a rotational speed and an actual rotational speed of the ultrasonic motor may vary among individual ultrasonic motors, due to variations in material constants such as in connection with piezoelectric ceramic and metal used for the vibrator 3 and the rotor 6, as well as due to tolerance variations in connection with assembling the vibrator 3 and the rotor 6. Such variations in the relationship between the indicated rotational speed and the actual rotational speed among the ultrasonic motors will detrimentally affect accurate control of the rotational speed and stable rotation of the ultrasonic motor 700.
Furthermore, in order to reduce driving power consumption, a voltage level of the DC power supply 25 is set at a minimum voltage which is capable of driving the ultrasonic motor 700 at a maximum rotational speed determined by the specification under a standard load. The driving range of the ultrasonic motor 700 is limited by the predetermined voltage level of the DC power supply 25 which supplies power to the power amplifiers 24 and 26. When the load on the ultrasonic motor 700 increases, a frequency of the driving signal will be reduced in order to obtain more power for driving the ultrasonic motor 700. Nevertheless, the driving frequency of the ultrasonic motor 700 cannot be shifted in order to increase the rotational power beyond the driving range which is limited by the voltage level of the DC power supply 25.
In a case where the voltage level of the DC power supply 25 is increased in order to increase the available driving power, the driving of the ultrasonic motor 700 is predominantly controlled by the driving voltage rather than the frequency of the driving signal. This makes it difficult to set the vibrator 3 to a low impedance state by shifting the frequency of the driving signal. Accordingly, the mechanical branch current does not flow efficiently, resulting in an increase in power consumption of the ultrasonic motor 700 under the standard load.
As shown in FIG. 21, a characteristic curve illustrating the relationship between the rotational speed and the driving frequency of the ultrasonic motor 700 exhibits hysteresis with respect to a sweeping direction of the driving frequency.
FIG. 22 shows the admittance characteristics of the vibrator 3 in a driving state as indicated by a curve 80, and in a stationary state (or when the ultrasonic motor 700 is stopped) as indicated by a curve 81. A resonance frequency of the vibrator 3 in the stationary state is higher than a driving frequency range of the vibrator 3 in the driving state. A changing ratio of the admittance curve 81 is very high (i.e. unstable) at frequencies lower than the resonance frequency. In addition, a frequency range which is capable of being driven is very narrow in the stationary state. Accordingly, in order to start driving the ultrasonic motor 700 stably and to drive it efficiently, it is required to sweep the driving frequency from a frequency sufficiently higher than the resonance frequency of the vibrator 3, when the ultrasonic motor 700 is started driving. Nevertheless, the conventional driving circuit 720 does not always start driving by sweeping the driving frequency from the higher side of the resonance frequency. Therefore, the conventional driving circuit 720 cannot start driving the ultrasonic motor 700 and drive it efficiently.
In the driving circuit 720 (FIG. 20), the rotating ultrasonic motor 700 is stopped by cutting off the supply of the driving signal 721, so that the vibrator 3 immediately comes to stop. However, the rotor 6 cannot stop immediately because of inertia of the rotor 6 itself, the transmission axis 7, an external load, and the like. Accordingly, a slip between the vibrator 3 and the rotor 9 occurs so as to generate audible noise and undesirable vibration, resulting in difficulty in driving the ultrasonic motor 700 silently.